1. Field of the Invention
The present invention relates to an intake system for an internal combustion engine, comprising first and second intake valve bores provided to face a combustion chamber and to be independently opened and closed by first and second intake valves, and first and second intake ports which are respectively connected to the first and second intake valve bores with a partition wall interposed therebetween and which communicate with a common intake passage, in which, in a particular operating range of the engine, an imbalance is produced in the concentration of the air-fuel mixture flowing through the first and second intake valve bores into the combustion chamber on the basis of a difference between the amounts of fresh air flowing into the first and second intake ports.
2. Description of the Prior Art
Such systems are conventionally known, for example, from Japanese Patent Application Laid-open No. 94407/92.
In the system disclosed in the above-described patent application, one of the pair of intake valves is opened and closed in a smaller amount and for a smaller time period than those of the other intake valve in a particular operating range of the engine, thereby producing a swirl required for lean burning. In such case, a difference is produced between the amounts of fresh air flowing into the intake ports, on the one hand, and in order to ensure that the fuel is substantially equally injected from a fuel injection valve into both the intake ports, an imbalance is produced in concentration of the air-fuel mixture flowing through the intake valve bores into the combustion chamber, on the other hand. More specifically, the concentration of the fuel in the air-fuel mixture flowing into the combustion chamber through the one intake valve bore opened in the smaller lift amount and for the smaller time period is higher than that of the fuel in the air-fuel mixture flowing into the combustion chamber through the other intake valve bore. Since the air-fuel mixture having the higher fuel concentration flows into the combustion chamber in the middle of an intake stroke, the concentration of the fuel in the vicinity of the spark plug is higher, thereby providing an improved ignitability for the lean burning, but resulting in a deteriorated nature of the exhaust gas and, in particular, resulting in NOx being liable to be produced.
On the other hand, there is a conventionally known intake system in which, as disclosed in Japanese Utility Model Publication No. 33852/84, in a particular operating range of the engine, only one of the pair of intake valves is opened and closed, while the other intake valve is brought into a completely inoperative state, thereby producing a swirl flow in the combustion chamber to cause a lean burning, without a problem of an imbalance in the concentration of the air-fuel mixture flowing through the intake ports into the combustion chamber. In this system, in order to prevent fuel droplets and the air-fuel mixture from being stagnated within the intake port which is in the inoperative state, the intake ports of the pair are connected to each other through a communication hole. However, if the construction is such that the air-fuel mixture is supplied only through the one intake port into the combustion chamber, an intensive swirl is produced within the combustion chamber and hence, a fuel concentration profile within the combustion chamber is such that the fuel concentration is gradually higher toward the radially outer portion of the combustion chamber. As a result, a reduction in ignitability during the lean burning or the like is brought about due to a relatively low fuel concentration around the spark plug located at substantially the center of the combustion chamber ceiling.